In power generation, at a specified output, an increase of the rotary speed of a turbine is typically associated with a decrease in size and costs. Efficiency, too, can be improved. Already, power generation turbines up to 70 MW are connected to generators by way of gearing arrangements, so as to allow operation at higher rotary speeds. As the output increases, the use of gearing arrangements becomes increasingly difficult for safety reasons. In such cases, the turbine is operated at synchronous speed.
The use of a gearing arrangement is typically associated with the following disadvantages:                a fixed transmission ratio;        a noise level above 100 db for 40 MW, and above 115 db for 70 MW;        mechanical losses irrespective of the particular load; and        exacting requirements with regard to cooling and lubrication with oil.        
The use of static frequency converters in the form of rectifier/inverter or the use of cycloconverters (power electronics) represents an alternative. The following advantages could be expected:                reduced costs of the generator in agreement with a constant product of volume and rotational speed;        a standardised generator for both 50 and 60 Hz;        an adjustable speed which allows restoration of the partial-load efficiency of the turbine;        reduced losses in relation to the gearing arrangement, at least in partial load;        a substantial reduction in noise;        clean (oil-free) cooling;        no upper limit of the possible output, resulting in a significant reduction in the cost of the turbine by keeping it small—an option not provided by a gearing arrangement; and        use of the generator as a starter motor (in the case of gas turbine applications).        
Both in the case of power generation and in the case of drives, a reduction in losses of the static frequency converters or cycloconverters would bring about substantial cost savings. A reduction of the losses would above all have a bearing on investment costs because cooling accounts for a substantial part of the total costs of the converter.
Furthermore, reduced cooling requirements provide the option of keeping the electronics more compact, thus facilitating integration of the power electronics in the electric power station or even in the generator unit. Close integration of the power electronics in the generator unit would provide the additional advantage of short connection lines, shared coolant devices and a smaller overall volume (savings in building costs).
In the field of large drives of up to several 10 MW, these advantages also arise from the reduced losses, thus providing a competitive advantage compared to direct mechanical drives of a turbine.
The indirect conversion which is used in rectifier/inverters (AC/DC/AC) is caused by generating a directed direct current or a directed direct voltage from the three-phase source (mains in the case of motors; generator in the case of power generation). Subsequently, the direct current or the direct voltage is converted back to an alternating current by means of an inverter.
An inductance (current converter) or a capacitor bank (voltage converter) are switched into the intermediate circuit so as to reduce the ripple component of the current or the spikes.
These days, rectifier/inverters make use of thyristors. If natural commutation of the thyristors is possible, the losses in the converter are reduced. However, induction motors for example, take up reactive power. In order to make this reactive power from the net available, it should be possible to switch off the current in a specified arm of the converter at any desired time. In this case there is forced commutation and thus there are increased losses. In the electrical machine (generator or motor), the phase currents are chopped direct currents. The armature reaction does not rotate at constant speed and amplitude but instead jumps around according to the commutation cycle. A 6-pulse or 12-pulse converter provides six or twelve different angular positions for the armature reaction. This results in strongly pulsating torques and large additional losses in the electrical machine which can lead to deterioration of the machine. In 12-pulse converters the effect is 4 times smaller than in 6-pulse converters.
Voltage converters use GTOs with their inherent high switching losses, as well as IGBTs or IGCTs. The power of the individual components is less than that of thyristors, consequently, a larger number of components are required for a specified voltage or a specified current. Voltage converters can benefit from the use of pulse-width modulation techniques which improve the shape of the current curves and reduce the harmonics. The higher the switching frequencies the better, except with regard to losses and dielectric fatigue. The curve shape of the current can largely be sine-shaped so that a decrease of power of the electrical machine is avoided.
Direct conversion (AC/AC) is for example possible by means of a so-called cyclo-converter. Direct conversion provides significant advantages from the point of view of the electrical machine, because the current is more or less a sine-shaped wave rather than chopped direct current. It reduces the losses which occur additionally within the electrical machine and it also prevents pulsating torques.
However, the use of cyclo-converters limits the achievable frequency range to 0–⅓ of the input frequency. Due to imbalanced operation, exceeding the ⅓ limit results in overdimensioning up to a factor of 3.
Another possibility of direct conversion is provided by a so-called matrix converter in which each phase of a multi-phase source (generator or mains) is connected or connectable with each phase of a multi-phase load (mains, passive load, motors, etc.) by a bi-directional switch (see e.g. N. Mohan et al., Power Electronics, 2nd Edition, John Wiley & Sons, New York pp 11–12). The switches consist of an adequate number of thyristors to withstand the differential voltage between the phases, and the phase currents, and to allow current reversal. They can be regarded as truly bi-directional components with the options of jointly using additional wiring such as snubbers or the power supplies for the drive pulses for the antiparallel components.
The switches are arranged in an (m×n)-matrix at m phases of the source and n phases of the load. This provides the option of establishing any desired connections between the input phases and the output phases; however at the same time it has the disadvantage in that certain switching states of the matrix must not be allowed since otherwise for example a short circuit would result. Furthermore it is desirable to carry out commutation from one phase to another phase such that the lowest possible switching losses result.
U.S. Pat. No. 5,594,636 describes a matrix converter and a process for its operation in which commutation between the phases is partly carried out as a natural commutation, with a forced commutation where natural commutation is not possible. Although with this type of selection, switching losses are reduced due to natural commutation, those switching losses which arise from forced commutation still remain. Furthermore, the possible forced commutation necessitates the use, in all positions on the matrix, of components which can be switched off. This considerably increases the switching expenditure.
However, it is possible to operate a matrix converter in a way that only natural commutations are being used. This can be achieved by only allowing the switching over from a selected connected phase of the generator to a selected not connected phase of the generator only if certain conditions are met. However, this mode of operation allowing a cheap and reliable control of the matrix converter can only be used to control frequency but not to control the voltage. Voltage is therefore controlled by means of the excitation system, as usual in large power generation. This voltage control strategy does not cause any problem when the rotating machine is operated in generating mode. But when the rotating machine is used as a motor, torque and current control, at low speed, requires a real voltage modulation, in-line with the so-called constant V/f rule.
When the prime mover of the power generation unit is a gas turbine, it is a usual practice to use a static frequency converter, i.e. rectifier/inverters, to operate the generator as a motor to start-up the shaft. The gas turbine does not produce a positive torque before it has reached 40–50% of the rated speed. So while to provide a matrix converter based on fully natural commutation has various advantages, it still necessitates to provide additional means to start-up the gas turbine unit in particular in a range between 0 to 20% load, where the so-called constant V/f rule has to be met. The strong attractiveness of the naturally commuted converter, in terms of cost, compactness and efficiency, is consequently reduced by the need for an additional starting device.